The present invention is related to power and wavelength monitoring for optical signals, and more specifically to monitoring the wavelength and power of the component optical signals of a multiplexed optical signal.
Dense Wavelength Division Multiplexing (DWDM) of optical signals has become a popular method to increase transmission bandwidth over existing fiber-optic backbones. In DWDM operation, multiple signal sources having different wavelengths share the same fiber transport system. In effect, the DWDM technology allows a single fiber to function as a plurality of fibers. The typical DWDM spectrum in normal operation is comprised of nearly uniformly spaced spectral components of nearly equal powers. For example, these signal sources often share the same optical fiber with spacing of only 100 GHz, 50 GHz, or even less, between signals within the 1528 to 1565 nm wavelength range defined by the principal gain region of an erbium doped fiber amplifier (EDFA), a typical component of a telecommunication system. Assuming this operating range, the spacing between these component signals is approximately 0.8 nm, 0.4 nm, or even less, respectively.
Semiconductor lasers are commonly used as the signaling sources for telecommunication systems utilizing optical signals. A typical semiconductor laser can be operated in a range of wavelengths depending upon its operating current and temperature. Even at a fixed temperature and current, it is expected that over time, e.g. several years, the wavelength of the light emitted from the laser will gradually shift or drift from the desired operating wavelength to a wavelength that is no longer suitable for the signal""s particular wavelength channel assignment. The power output of a laser can also vary over time, often by as much as a factor of ten over approximately a decade of use. By observing the wavelength shift or drift and the output power of an optical signal, the laser performance may be corrected by adjusting the temperature and/or current of the semiconductor laser to maintain the semiconductor laser at desired operating parameters.
Monitoring the wavelength and power of these optical signals, therefore, has become increasingly important as wavelength spacing decreases between the component optical signals of multiplexed optical signals. Commercial optical spectrum analyzers are available for analyzing the wavelength and power levels of component signals of multiplexed optical signals. For example, a grating spectrometer or monochromator disperses optical signals in one dimension onto a detector. The mechanism may be motorized to scan the wavelengths for display on a Cathode Ray Tube (CRT) or strip chart recorder. A grating spectrometer usually has moderate dispersion and so the resolution is also moderate. It is able to cover a significant range of wavelengths without ambiguity because of its large free spectral range.
A scanning Fabry-Perot interferometer may also be used as a spectrum analyzer. This analyzer includes a tunable, narrow-band filter. The optical transmission may also be displayed on a CRT. This device usually has relatively high resolution because it is used at a high order, but consequently the device has a small free spectral range. The analyzer is associated with a figure of merit, its xe2x80x9cfinessexe2x80x9d which is approximately the ratio of its resolution to its free spectral range. The finesse is related to the reflectivity of the two mirror surfaces utilized by the device, approximately the reciprocal of the fraction of optical loss for a round-trip inside the optical cavity of the interferometer. The lack of a proper optical signal due to poor flatness or parallelism causes the light to deviate from its multiple ideal round-trip path, thereby contributing to a reduction in finesse. Most Fabry-Perot interferometers for high resolution spectral scanning use piezoelectric transducers to mechanically change the mirror-to-mirror separation.
Another functional spectrometer is a Fourier transform spectrometer that takes the output of a scanned two-beam interferometer and calculates the Fourier transform of the signal as a representation of the spectral components of the aggregated optical signal. Again, a motor is used to mechanically drive the change in path length difference of the spectrometer.
All of the above-described spectral analysis methods and devices require the use of mechanical motion to determine the optical spectrum of an optical signal. Monitors have been proposed that do not rely on mechanical motion, such as U.S. Pat. No. 5,850,292 to Braun for a xe2x80x9cWavelength Monitor For Optical Signals,xe2x80x9d the entirety of which is incorporated herein by reference, but it is still desirable to have an optical wavelength and power monitor that may be integrated into an optical telecommunication system in a cost-effective manner and which can effectively determine the wavelengths and powers of component optical signals of a multiplexed optical signal without reliance on moving parts.
The present invention is a method and apparatus for determining the wavelength and power of component optical signals of a multiplexed optical signal. The apparatus includes an optical waveguide router and an array of optical detectors. The optical waveguide router includes an input star coupler with at least one input waveguide and a plurality of output waveguides, a plurality of grating arms optically connected to the output waveguides of the input star coupler having path length differences between adjacent grating arms and, and an output star coupler having a plurality of input waveguides optically connected to the grating arms and at least twice as many output waveguides as the number of component optical signals. The array of optical detectors includes a plurality of optical detectors disposed to detect output optical signals from the output waveguides of the output star coupler. The optical detectors produce a plurality of electrical output signals corresponding to the power of each output optical signal. The apparatus also includes a means for comparing the electrical output signal to a predetermined set of output responses, a means for determining the wavelengths of the component optical signals from the comparison, and a means for determining the powers of the component optical signals from the comparison, each of which is preferably computer implemented.
The apparatus for determining the wavelength and power of the component optical signals may be easily incorporated into an optical transmitter system of a telecommunication system. Further, the apparatus is cost efficient and requires no mechanical motion to evaluate the optical spectrum of a multiplexed optical signal.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings.